The most common current use of ultra-capacitor cells for hybrid energy storage systems (HESS) is in transit busses. These system are offered as modular systems with a typical bus having one ½ kW-hr storage module. Bluways in Europe offers a 700 Volt ½ kW-hr module for $45,000, which is a cost per kW-hr of $90,000. A city bus weighs approximately 15 tons and will typically be stopping from 40 mph or less. On the other hand a typical commuter train with 6 bi-level coach cars will weigh 600 tons and should be stopping from 79 mph. The train has 160 times more kinetic energy. Calculations indicate this train will require 80 kW-hr's of ultra-capacitors to capture that energy and at $90,000/kW-hr that would cost $7.2 million which is more than the $6.3 million that a new Tier 4 diesel passenger locomotive costs. Existing UCESS modules for transit buses would not package well on a rail tender car and would also not stand up well to the more constant steel wheel on steel rail vibration over the 20 year locomotive service life.
The commuter rail application has several unique characteristics that make it an excellent opportunity to utilize the benefits of a HESS. Unlike typical on road applications such as diesel trucks, a diesel electric locomotive already has the electrical machinery to capture the braking energy. With the reduced rolling friction of steel wheels on steel rails, the impact of the additional weight of a HESS is minor compared to that of a rubber tired on-road vehicle. This overcomes the battery advantage over ultra-capacitors of higher energy density.
Switcher locomotives are another application where hybridization would be beneficial due to the frequent start and stopping action of the locomotive as it moves small strings of cars back and forth to build up or take apart longer trains.
What is desired is a novel packaging and control system for ultra-capacitor systems that decreases the per kW-hr cost while adding system redundancy and reliability. This system should be backwards compatible as much as possible with existing locomotives and locomotive control systems. It should also be able to incorporate both ultra capacitor cells and battery cells in a modular system with adequate provisions to insure safe operation